build: drop the -Wswitch-enum warning

-Wswitch-enum is a bit awkward if a switch statement is intended to
handle just some of the named codes of an enumeration especially, and
leave the rest to the default label.

We already have -Wall, which enables -Wswitch by default, and per GCC
documentation, "The only difference between -Wswitch and this option
[-Wswitch-enum] is that this option gives a warning about an omitted
enumeration code even if there is a default label."

Drop -Wswitch-enum to not force listing all named codes of
enumerations in switch statements that have a default label.